I like seeing Mary and Rosalie in this: I feel as if everyone forgets later on that Joey and Rosalie have known each other since well before CS times. It also explains a lot about Madge's interest in teaching, Grizel's affection for the Bettanys, and Madge's friendship with Mlle. Some of it - the sphinx bit! - did drag on a bit, but some of the CS plays do that too
We're told very little about Madge and Joey's background in the canon books - we don't even get the names of "the aunts", and they vanish off the face of the earth very early on! - so I definitely think there was room for those gaps to be filled in. It's one person's interpretation of how things might have been, but I think it works very well. I don't think there's anything that really jars or feels as if it's something that doesn't fit.